There are many applications for spin-on filters. Typically, the filter includes a cannister which contains a filter element and has a face provided with a central internally threaded port for screw on attachment to a threaded male port on a header cover casting. A second port, generally a plurality of second ports, is located radially outwardly of the internally threaded port and is alignable with an annular port on the cover casting. An annular seal is disposed on the face of the filter and has an axially facing seal surface centered about the internally threaded port and located outwardly of the second port to be engageable with an annular, axially facing seat surrounding the annular port of the cover casting. In the usual case, the seal is located in an axially opening groove in the face of the filter and extends somewhat out of the same. When the filter is being screwed on the threaded male port of the cover casting, upon contact of the seal with the annular seat, the seal becomes compressed to establish good sealing engagement between the seal and the seat.
These seals work well for their intended purpose. However, in some instances, replacement of the filter may pose a problem in terms of leakage of the fluid being filtered from the filter-cover casting interface, which leakage may be under pressure.
In particular, where such filters are employed in valve systems, the closing of the valves preliminary to removing of the filter frequently results in a trapped body of fluid between the valves and within the filter, which trapped body of fluid is under pressure and is maintained under pressure because of slight expansion of system components in response to such pressure when the valves were opened.
When the filter is unscrewed from the cover casting, almost immediately the sealing engagement between the seal on the filter and the seat on the cover casting is lost. Residual pressure within the system then causes the fluid to squirt through such interface, an undesirable occurrence.
One attempt to solve this problem is illustrated in a U.S. Pat. No. 3,685,659 issued Aug. 22, 1972, to Heskett, et al. Heskett employs a separate valve as part of his system construction which may be operated to pass a small amount of fluid to an external receiving chamber to relieve pressure prior to removal of a filter bowl from a filter head. Though this approach is operative, it first of all requires a conscious effort on the part of the user of the system to utilize the valve to dissipate pressure prior to filter bowl removal or else the pressure will not be dissipated. Consequently, in the case of a forgetful operator, the Heskett, et al, system cannot provide this intended and desirable function.
Another disadvantage of the Heskett system is the fact that its fabrication requires the use of a valve element along with a special chamber. These components add to the complexity of the system, therefore substantially increasing the cost thereof.
The present invention is directed to overcoming the above problem.